The present invention is directed to a control assembly for use in conditioned air distribution systems of the type wherein air, conditioned at a central source, is distributed to a plurality of different rooms or areas within a room. More particularly, this invention is directed to an improved variable volume control assembly for use in conditioned air distribution systems.
Air is used as the medium of thermal transfer in all forced air heating and air conditioning systems. In such systems, air is heated and/or cooled at a central source and the conditioned air (hereinafter inclusive of heated or cooled air) is distributed to a plurality of zones through a system of ducts and outlets. There are generally two different methods for controlling the flow of conditioned air, the constant volume method and the variable volume method. In the constant volume method, the flow of conditioned air into any zone is somewhat constant but the temperature of the conditioned air is varied by either changing the source of air or by mixing hot and cold air. This constant volume system thus requires a constant source of different types of conditioned air. This system generally requires dual ducting and also requires a means for controlling the mixing of the hot air and cold air. This type of blending consumes excessive energy. The variable volume method, on the other hand, employs conditioned air at a somwhat constant temperature but varies the volume of conditioned air delivered to any particular zone in response to the demand of the zone.
As is known in the art, the variable volume system offers many advantages over the use of the constant volume system. The variable volume system requires only one source of conditioned air at a somewhat constant temperature while the constant volume system requires two sources of thermal energy. The variable volume system requires only a single duct system while the constant volume system requires a dual thermal energy system with a means for controlled mixing of the different thermal levels of conditioned air. The variable volume system is advantageous for heating or cooling the interior of standard office buildings, particularly perimeter or exterior facing rooms. Larger energy savings are possible with variable volume systems than other means of controlling the heating and cooling of a building.
Even though the variable volume system is preferred, it is not without its own drawbacks. Humidity control of the air is not as good as blending types of systems. Variable volume systems can be reduced to very low room air changes and where heavy smoking is present, an insufficient air change can occur. Most terminal control assemblies used in the variable volume system are designed to operate within a certain volume range and operation above this range causes undesirable levels of noise and, in fact, operation within the range usually causes considerable noise. Present variable volume terminal assemblies have high pressure loss values and require excessive fan horsepower to deliver the desired volume of air. Additionally, the air flow pressure sensing means used in existing variable volume control assemblies is not as sensitive as desired and does not offer pressure independent control over a wide range of flow rates.